1. Field of the Invention
The present invention relates to a liquid crystal display device using a liquid crystal panel.
2. Related Art
A liquid crystal panel switches display levels of a picture by switching electrical signals from outside. The liquid crystal panel usually receives display data from outside, converts the display data into electrical signal waveforms of predetermined voltage levels, and applies the waveforms to the liquid crystal elements to thereby display a picture. The switching time of the display levels by the liquid crystal panel is some 10 ms in general.
FIG. 7 illustrates an example of response waveform when an electrical signal is applied to one liquid crystal element. In FIG. 7(a), an electrical signal waveform 101 illustrates a voltage applied to the liquid crystal element. The horizontal axis represents the time and the vertical axis represents the signal level. T1 represents a period of time during which a low signal level 1 is supplied. T2 represents a period of time during which a high signal level 2 is supplied. In FIG. 7(b), an optical response waveform 102 represents an optical response waveform of the liquid crystal element when the voltage of the electrical signal waveform 101 is applied, the horizontal axis represents the time corresponding to the electrical signal waveform, and the vertical axis represents the level of light transmittance of the liquid crystal element. The light transmittance shows a low level 1 during the period T1, and a high level 2 during the period T2.
When the signal level of the electrical signal waveform 101 is switched from the low signal level 1 into the high signal level 2, a transitional period T3 appears between the period T1 and the period T2, due to the delay in the optical response of the liquid crystal element. When the light transmittance level 1 is 0%, and the light transmittance level 2 is 100%, the time required for the light transmittance varying from 10% to 90% is called the rise response time or response speed of the liquid crystal panel. Also, when the signal level is switched from the high signal level 2 into the low signal level 1, the time required for the light transmittance varying from 90% to 10% is similarly called the fall response time of the liquid crystal panel.
Usually, the liquid crystal display device is driven by TV signals or signals from the personal computers, and the display switching frequency of these signals are about 60 Hz in general. This means that the display period of one picture screen is approximately 16.7 ms, and the picture screen is always rewritten by this time interval. On the other hand, the response speed of the liquid crystal element of the liquid crystal panel is around 30 to 60 ms in general. Therefore, the response is not complete within the time equivalent to the display switching frequency of 60 Hz.
As a method of improving such slowness in the response time of a liquid crystal panel, for example, a method of accelerating the movement of a liquid crystal element by temporarily applying a higher or lower signal level at the switching time of a drive signal of the liquid crystal panel has been proposed in the Japanese patent No. 2616652. This method is a technique applied to a liquid crystal panel of the Super Twisted Nematic (STN) mode, wherein the response speed of the liquid crystal element is rather slow as some 100 ms. This technique is effective for the liquid crystal panels with slow response speeds, for example, when the response waveform of the drive electrical signal waveform and the liquid crystal panel using this technique will improve the response speed from an optical response waveform 102 to an optical response waveform 102a as shown in FIG. 8. FIG. 8(a) illustrates the signal level variation of the electrical signal waveform 101, and FIG. 8(b) illustrates the optical response waveforms 102, 102a that represent the variations of the light transmittance corresponding to the signal level variation. xe2x80x9cAxe2x80x9d in FIG. 8(a) represents the level of compensation, and the waveform 102 in FIG. 8(b) shows the optical response waveform when not compensated, and the waveform 102a shows the optical response waveform when compensated.
However, when this technique is applied to a liquid crystal panel with a comparably high response speed of the liquid crystal element as some 10 ms, such as the recent TFT-LCD using a thin film transistor, for example, when the electrical signal waveform 101 is switched from the low voltage level into the high voltage as shown in FIG. 9(a), there is a possibility that an overshooting occurs in the optical response waveform 102, as shown in FIG. 9(b), which is not desirable in terms of visibility of the display. On the other hand, when the voltage level is switched from the high level to the low level, there is a possibility that an undershooting occurs.
Furthermore, when the signal level after switching the display reaches the highest level, the compensation signal with a level higher than this highest level cannot be generated, which leads to a problem that disables improvement of the response speed.
According to one aspect of the invention, the liquid crystal display device includes: a delay circuit that delays an inputted current display data by one display period and outputs a delayed display data; a reference unit that possesses a reference data containing a plurality of reference values corresponding to both the current display data and the delayed display data, and outputs an output data based on a reference value selected from the plurality of the reference values; and a liquid crystal panel that receives a supply of video signals on the basis of the output data. Further, the reference value selected when the current display data changes from the delayed display data is set so that an optical response of the liquid crystal panel in relation to the change is substantially completed within the one display period.
Thus, the liquid crystal device of the invention is provided with a reference circuit that possesses a reference data containing a plurality of reference values corresponding to both the current display data and the delayed display data, and with regard to all the combinations of the current display data and the delayed display data, the data are prepared in advance, with which the optical response of a liquid crystal element is completed within the one display period neither too much nor too less. Thereby, the optical response of the liquid crystal element of the liquid crystal panel can be completed within the one display period without generating an overshooting or an undershooting at any data level changes.
According to another aspect of the invention, in the liquid crystal display device, the level of the output data is compressed by a specific depth in such a manner that the level of the output data when the delayed display data and the current display data have the same value becomes lower than the level of the current display data, and the reference value selected when the current display data becomes higher than the delayed display data is set to expand the output data within the specific depth.
According to this, if the level of the current display data has a maximum level, it is possible to expand the output data within the specific depth, whereby the optical response of the liquid crystal element of the liquid crystal panel can be completed within the one display period.
According to another aspect of the invention, in the liquid crystal display device, the level of the output data is compressed by a specific depth in such a manner that the level of the output data when the delayed display data and the current display data have the same value becomes lower than the level of the current display data, and a lowering of a luminance of the liquid crystal panel based on the compression of the output data is compensated by raising the luminance of a backlighting of the liquid crystal panel.
According to this, by compensating the lowering of the luminance due to the level compression of the display data with the backlighting, the optical response of the liquid crystal element of the liquid crystal panel can be completed within the one display period, and also the display state with a desirable luminance can be achieved.